Co-evolution is when two different things, like animals or plants, change over time because of how they interact with each other. It's kind of like a dance where one partner makes a move and the other partner has to respond in a certain way.
Imagine a flower and a bee. The flower needs the bee to help it spread its pollen to other flowers, so it can make seeds and have babies. The bee needs the flower for the sweet nectar it makes, which is like a yummy drink for the bee. Over time, the flower and the bee change together, so they can work together better.
For example, the flower might start to make a shape that's perfect for the bee to land on and drink from. The bee might get better at finding that shape and knowing when the flower is ready to make new seeds. Their relationship gets better and better over time, like they're learning to do a dance together perfectly.
Co-evolution happens all the time in the natural world, and it's a really cool and important part of how living things survive and thrive.